[DwarfDebug] Add forward declarations of "<" operators [NFC]

The operators are defined in DwarfDebug.cpp but are
referenced in the struct definitions of FrameIndexExpr and
EntryValueInfo in DwarfDebug.h, and since they weren't
declared before, gcc warned with

 [694/5646] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/DwarfDebug.cpp.o
 ../lib/CodeGen/AsmPrinter/DwarfDebug.cpp:273:6: warning: 'bool llvm::operator<(const llvm::FrameIndexExpr&, const llvm::FrameIndexExpr&)' has not been declared within 'llvm'
   273 | bool llvm::operator<(const FrameIndexExpr &LHS, const FrameIndexExpr &RHS) {
       |      ^~~~
 In file included from ../lib/CodeGen/AsmPrinter/DwarfDebug.cpp:13:
 ../lib/CodeGen/AsmPrinter/DwarfDebug.h:112:15: note: only here as a 'friend'
   112 |   friend bool operator<(const FrameIndexExpr &LHS, const FrameIndexExpr &RHS);
       |               ^~~~~~~~
 ../lib/CodeGen/AsmPrinter/DwarfDebug.cpp:278:6: warning: 'bool llvm::operator<(const llvm::EntryValueInfo&, const llvm::EntryValueInfo&)' has not been declared within 'llvm'
   278 | bool llvm::operator<(const EntryValueInfo &LHS, const EntryValueInfo &RHS) {
       |      ^~~~
 In file included from ../lib/CodeGen/AsmPrinter/DwarfDebug.cpp:13:
 ../lib/CodeGen/AsmPrinter/DwarfDebug.h:121:15: note: only here as a 'friend'
   121 |   friend bool operator<(const EntryValueInfo &LHS, const EntryValueInfo &RHS);
       |               ^~~~~~~~

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h

@@ -103,6 +103,11 @@ class DbgEntity {
 
 class DbgVariable;
 
+bool operator<(const struct FrameIndexExpr &LHS,
+               const struct FrameIndexExpr &RHS);
+bool operator<(const struct EntryValueInfo &LHS,
+               const struct EntryValueInfo &RHS);
+
 /// Proxy for one MMI entry.
 struct FrameIndexExpr {
   int FI;